The human spine is comprised of individual vertebrae 30 that are connected to each other to form a spinal column 29, shown in FIG. 1A. Referring to FIGS. 1B and 1C, each vertebra 30 has a cylindrical bony body (vertebral body) 32, three winglike projections (two transverse processes 33, 35 and one spinous process 34), left and right facet joints 46, lamina 47, left and right pedicles 48 and a bony arch (neural arch) 36. The bodies of the vertebrae 32 are stacked one on top of the other and form the strong but flexible spinal column 29. The neural arches 36 are positioned so that the space they enclose forms a tube, i.e., the spinal canal 37. The spinal canal 37 houses and protects the spinal cord and other neural elements. A fluid-filled protective membrane, the dura 38, covers the contents of the spinal canal. The spinal column is flexible enough to allow the body to twist and bend, but sturdy enough to support and protect the spinal cord and the other neural elements. The vertebrae 30 are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs 40. Disorders of the spine occur when one or more of the individual vertebrae 30 and/or the inter-vertebral discs 40 become abnormal either as a result of disease or injury. In these pathologic circumstances, fusion of adjacent vertebral segments may be tried to restore the function of the spine to normal, achieve stability, protect the neural structures, or to relieve the patient of discomfort.
Several spinal fixation systems exist for stabilizing the spine so that bony fusion is achieved. The majority of these fixation systems utilize rods that attach to screws threaded into the vertebral bodies or the pedicles 48, shown in FIG. 1C. In some cases component fixation systems are also used to fuse two adjacent vertebral segments. The component fixation systems usually include two longitudinal components that are each placed laterally to connect two adjacent pedicles of the segments to be fused. The longitudinal components may be plates, rods, and wires, among others. This system can be extended along the sides of the spine by connecting two adjacent pedicles at a time, similar to the concept of a bicycle chain. Current component fixation systems are designed to function in place of rods with the advantage of allowing intersegmental fixation without the need to contour a long rod across multiple segments. Both the component systems and the rod systems add bulk along the lateral aspect of the spine and limit access to the pars and transverse processes for decortication and placement of bone graft. In order to avoid this limitation, many surgeons decorticate before placing the rods. However, decortication is not always desirable because it increases the amount of blood loss and makes it more difficult to maintain a clear operative field. Placing rods or components lateral to the spine leaves the center of the spinal canal that contains the dura, spinal cords and nerves completely exposed. In situations where problems develop at the junction above or below the fused segments additional fusion may be necessary. However, the rod fixation system is difficult to extend to higher or lower levels that need to be fused. Although there are connectors and techniques to lengthen the fixation via rods, they tend to be complex, difficult to use, and time consuming.
It is desirable to have a spinal stabilization device that does not add bulk to the lateral aspect of the spine, is extendable, and does not limit access to the pars and transverse processes for decortication and placement of bone graft.